352 
ROBERT AARON STEINBERG 
without addition of either phosphoric acid or ammonium hydroxide com- 
pares well with the value (0.09 mg. Zn/L) found in another experiment (65). 
The curve obtained in the presence of NH4OH is rather peculiar, and it 
may well be that the optimum should be taken in this case as in the neigh- 
borhood of 25 mg. Zn/L. 
Increased acidity and presence of zinc ions exercise the same effect on 
the growth of A. niger — a positive acceleration. The more zinc the lower 
the acidity (and vice versa) required for the production of the maximum 
yield. The data already given show this to be the case. I am inclined to 
infer, although the authors do not themselves suggest this interpretation, 
that the same relation appears to hold for the action of iron and increased 
acidity on the rice plant (Gile and Carrero, 18, p. 521, tables 13 and 14); 
and for the action of aluminum and increased acidity (barley and rye) 
studied by Hartwell and Pember (19). 
Not only the yields but also the fructification and the acidity attained 
in the cultures are correlated with the increased growth. In Figure 2 I 
have indicated by the letter "R" the lowest zinc concentrations exercising 
a perceptible effect on the sporulation of the cultures. These values are 
also contained in the following table (part of experiment 25), The hydrogen 
ion exponent (pn) of the solutions at the time of harvest has also been added 
(see below). 
Spore Formation in Pfeffer Solution 
Partially retarded 
+ 0.2 cc. NH40H/flask + i mg. Zn/L pn = 2-3 
+ 0.1 mg. Zn/L 1-2 
+ 0.2 cc. H3P04/flask 2-3 
Sterile, etc. 
+ 0.2 cc. NH40H/flask + 100 mg. Zn/L (vigorous) Ph ^ 3 
+ 100 mg. Zn/L (sparse) 1-2 
+ 0.2 cc. H3P04/flask + i.o mg. Zn/L (sterile) 1-2 
Sporulation is therefore retarded through increased acidity and within 
limits favored by a decreased acidity. The first indication of retardation 
of spore formation occurs at an acidity corresponding to a hydrogen exponent 
of 2-3. In the ammonium hydroxide series spore formation was vigorous 
even in the presence of 100 mg. Zn/L. Here the low acidity (pn < 3) is 
without doubt the correct explanation. The extent of the retardation of 
sporulation depends apparently on the acidity attained in the culture 
through the growth of the organism. 
Use was made of the indicator method as given by Michaelis (41) for 
the estimation of the above listed acidities. The following results (part of 
experiment 25) were also obtained in this manner. 
It will be noted that the cultures as a result of the growth of the organism 
attain an increased acidity in all cases, except in the Pfeffer solution without 
any additions and apparently in the Pfeffer solution to which phosphoric 
acid was added. 
